I wonder if a lot of people associate atheism with a bleak sort of nihilism, and fear that atheism means they'd be compelled to lose all belief in anything, good or bad. Atheism also got a lot of bad press in the first half of the 20th century, being associated with socialism and communism and a general sense of being opposed to all things godly and American and family picnics and mom's apple pie. And, frankly, some atheists are just assholes, which doesn't reflect well on the rest of us.
I rather resisted it myself for awhile, and called myself an agnostic. Truth is, though, I'm an atheis